A tour of the C# language

The C# language is the most popular language for the .NET platform, a free, cross-platform, open source development environment. C# programs can run on many different devices, from Internet of Things (IoT) devices to the cloud and everywhere in between. You can write apps for phone, desktop, and laptop computers and servers.

C# is a cross-platform general purpose language that makes developers productive while writing highly performant code. With millions of developers, C# is the most popular .NET language. C# has broad support in the ecosystem and all .NET workloads. Based on object-oriented principles, it incorporates many features from other paradigms, not least functional programming. Low-level features support high-efficiency scenarios without writing unsafe code. Most of the .NET runtime and libraries are written in C#, and advances in C# often benefit all .NET developers.

Hello world

The "Hello, World" program is traditionally used to introduce a programming language. Here it is in C#:

// This line prints "Hello, World" 
Console.WriteLine("Hello, World");

The line starting with // is a single line comment. C# single line comments start with // and continue to the end of the current line. C# also supports multi-line comments. Multi-line comments start with /* and end with */. The WriteLine method of the Console class, which is in the System namespace, produces the output of the program. This class is provided by the standard class libraries, which, by default, are automatically referenced in every C# program.

The preceding example shows one form of a "Hello, World" program, using top-level statements. Earlier versions of C# required you to define the program's entry point in a method. This format is still valid, and you'll see it in many existing C# samples. You should be familiar with this form as well, as shown in the following example:

using System;

class Hello
{
    static void Main()
    {
        // This line prints "Hello, World" 
        Console.WriteLine("Hello, World");
    }
}

This version shows the building blocks you use in your programs. The "Hello, World" program starts with a using directive that references the System namespace. Namespaces provide a hierarchical means of organizing C# programs and libraries. Namespaces contain types and other namespaces—for example, the System namespace contains many types, such as the Console class referenced in the program, and many other namespaces, such as IO and Collections. A using directive that references a given namespace enables unqualified use of the types that are members of that namespace. Because of the using directive, the program can use Console.WriteLine as shorthand for System.Console.WriteLine. In the earlier example, that namespace was implicitly included.

The Hello class declared by the "Hello, World" program has a single member, the method named Main. The Main method is declared with the static modifier. While instance methods can reference a particular enclosing object instance using the keyword this, static methods operate without reference to a particular object. By convention, when there are no top-level statements a static method named Main serves as the entry point of a C# program.

Both entry point forms produce equivalent code. When you use top-level statements, the compiler synthesizes the containing class and method for the program's entry point.

Familiar C# features

C# is approachable for beginners yet offers advanced features for experienced developers writing specialized applications. You can be productive quickly. You can learn more specialized techniques as you need them for your applications.

C# apps benefit from the .NET Runtime's automatic memory management. C# apps also use the extensive runtime libraries provided by the .NET SDK. Some components are platform independent, like file system libraries, data collections, and math libraries. Others are specific to a single workload, like the ASP.NET Core web libraries, or the .NET MAUI UI library. A rich Open Source ecosystem on NuGet augments the libraries that are part of the runtime. These libraries provide even more components you can use.

C# is in the C family of languages. C# syntax is familiar if you've used C, C++, JavaScript, or Java. Like all languages in the C family, semi-colons (;) define the end of statements. C# identifiers are case-sensitive. C# has the same use of braces, { and }, control statements like if, else and switch, and looping constructs like for, and while. C# also has a foreach statement for any collection type.

C# is a strongly typed language. Every variable you declare has a type known at compile time. The compiler, or editing tools tell you if you're using that type incorrectly. You can fix those errors before you ever run your program. Fundamental data types are built into the language and runtime: value types like int, double, char, reference types like string, arrays, and other collections. As you write your programs, you create your own types. Those types can be struct types for values, or class types that define object-oriented behavior. You can add the record modifier to either struct or class types so the compiler synthesizes code for equality comparisons. You can also create interface definitions, which define a contract, or a set of members, that a type implementing that interface must provide. You can also define generic types and methods. Generics use type parameters to provide a placeholder for an actual type when used.

As you write code, you define functions, also called methods, as members of struct and class types. These methods define the behavior of your types. Methods can be overloaded, with different number or types of parameters. Methods can optionally return a value. In addition to methods, C# types can have properties, which are data elements backed by functions called accessors. C# types can define events, which allow a type to notify subscribers of important actions. C# supports object oriented techniques such as inheritance and polymorphism for class types.

C# apps use exceptions to report and handle errors. You'll be familiar with this practice if you've used C++ or Java. Your code throws an exception when it can't do what was intended. Other code, no matter how many levels up the call stack, can optionally recover by using a try - catch block.

Distinctive C# features

Some elements of C# might be less familiar. Language integrated query (LINQ) provides a common pattern-based syntax to query or transform any collection of data. LINQ unifies the syntax for querying in-memory collections, structured data like XML or JSON, database storage, and even cloud based data APIs. You learn one set of syntax and you can search and manipulate data regardless of its storage. The following query finds all students whose grade point average is greater than 3.5:

var honorRoll = from student in Students
                where student.GPA > 3.5
                select student;

The preceding query works for many storage types represented by Students. It could be a collection of objects, a database table, a cloud storage blob, or an XML structure. The same query syntax works for all storage types.

The Task based asynchronous programming model enables you to write code that reads as though it runs synchronously, even though it runs asynchronously. It utilizes the async and await keywords to describe methods that are asynchronous, and when an expression evaluates asynchronously. The following sample awaits an asynchronous web request. When the asynchronous operation completes, the method returns the length of the response:

public static async Task<int> GetPageLengthAsync(string endpoint)
{
    var client = new HttpClient();
    var uri = new Uri(endpoint);
    byte[] content = await client.GetByteArrayAsync(uri);
    return content.Length;
}

C# also supports an await foreach statement to iterate a collection backed by an asynchronous operation, like a GraphQL paging API. The following sample reads data in chunks, returning an iterator that provides access to each element when it's available:

public static async IAsyncEnumerable<int> ReadSequence()
{
    int index = 0;
    while (index < 100)
    {
        int[] nextChunk = await GetNextChunk(index);
        if (nextChunk.Length == 0)
        {
            yield break;
        }
        foreach (var item in nextChunk)
        {
            yield return item;
        }
        index++;
    }
}

Callers can iterate the collection using an await foreach statement:

await foreach (var number in ReadSequence())
{
    Console.WriteLine(number);
}

C# provides pattern matching. Those expressions enable you to inspect data and make decisions based on its characteristics. Pattern matching provides a great syntax for control flow based on data. The following code shows how methods for the boolean and, or, and xor operations could be expressed using pattern matching syntax:

public static bool Or(bool left, bool right) =>
    (left, right) switch
    {
        (true, true) => true,
        (true, false) => true,
        (false, true) => true,
        (false, false) => false,
    };

public static bool And(bool left, bool right) =>
    (left, right) switch
    {
        (true, true) => true,
        (true, false) => false,
        (false, true) => false,
        (false, false) => false,
    };
public static bool Xor(bool left, bool right) =>
    (left, right) switch
    {
        (true, true) => false,
        (true, false) => true,
        (false, true) => true,
        (false, false) => false,
    };

Pattern matching expressions can be simplified using _ as a catch all for any value. The following example shows how you can simplify the and method:

public static bool ReducedAnd(bool left, bool right) =>
    (left, right) switch
    {
        (true, true) => true,
        (_, _) => false,
    };

Finally, as part of the .NET ecosystem, you can use Visual Studio, or Visual Studio Code with the C# DevKit. These tools provide rich understanding of C#, including the code you write. They also provide debugging capabilities.